The transmission is a very important part in speed change electric vehicles, electric hand tools and other applications. The whole speed change system typically represents about half of the production cost; therefore research on the speed change mechanism is always an important item for any company in these fields. A good transmission can change the rotational speed ratio according to different requirements. For use in an eclectic vehicle such as an electric bicycle, the transmission can be used to change the speed ratio so as to achieve the objective of an appropriate rotational speed or torque for the rider in different conditions.
Planetary gear sets are commonly used in the transmission of an electric vehicle. A planetary internal transmission uses a planetary gear train as the principal part of a speed change mechanism which is typically installed in a gear housing of a rear wheel of the vehicle. A planetary internal transmission achieves different rotational speed ratios by changing the gear ratio of the planetary gear train through different input parts, output parts and fixed parts.
The use of a planetary gear train as the principal part of a speed change mechanism in an internal transmission has the following advantages. First, the volume is small and the structure is precise, and there is little limitation to the narrow installation and usage space of the rear wheel hub. Second, the gearshift process works stably. When used in a vehicle in which the rear wheel is driven by a chain, there is no requirement for the chain to be moved to engage with different chain wheels to change a gear ratio, thus the drawback of chain separation is effectively improved, and the transmission efficiency is increased. Third, the speed change mechanism is protected by the external housing of the wheel hub and is not affected by the external environment, thus the usage lifetime is increased. Fourth, the speed change mechanism is installed inside the wheel hub, and there is not as much noise as an external transmission produces during the gearshift process.
Planetary gear sets are well known and are sometimes referred to as epicyclic gearing. Often such gear sets are used to provide a speed reduction, in which case they may be referred to in the market as an “Epicyclic Gear Reducer” or “Planetary Gear Reducer”. However, they can be arranged to provide a speed increase or overdrive. In a planetary gear set, there is a central gear typically called the “sun gear” which is surrounded by an annular, outermost gear called the “ring gear”. The sun gear has teeth on its outer periphery and the ring gear has teeth on its inner periphery facing the sun gear. A plurality of “planet pinions” are spaced about the sun gear and mesh with both the sun gear and the ring gear. The planet pinions are typically arranged in groups of three of four connected together by means of a carrier arm which keeps the planet pinions in position relative to one another. Each planet pinion rotates about its own axis and where the carrier arm is not fixed, the pinions can rotate as group together with the carrier arm about the sun gear.
According to the known transmission principle of the planetary gear train working as a transmission, the sun gear, the planet pinion carrier arm, and ring gear spin around one another. In different gear ratios, one of the three is fixed and does not spin, another is driven to provide an input, and the other is an output. Different combinations result in different gear ratios needed when reducing, increasing and/or reversing the rotational speed/direction. Six possible combination conditions are shown in Table 1 below.
TABLE 1Possible configurations of planetary gear set.CombinationCondition123456Sun GearFixedOutputFixedInputOutputInputPlanet pinionInputInputOutputOutputFixedFixedRing GearOutputFixedInputFixedInputOutput
There are a number of rules that govern how the various components of a planetary gear set operate. Thus, the sun gear and the planet pinions always rotate in opposite directions. Where the ring gear and the planet pinions are free to rotate at the same time; they always rotate in the same direction. When the planet pinion carrier arm is the output, it always rotates in the same direction as the input.
Additionally, environmental issues have significantly promoted the use of electric bicycles and vehicles in recent years. Most electric bicycles and vehicles in use today are equipped with internal transmissions. Thus, internal transmissions have gradually become the research key point of large scale vehicle manufacturers.
The present invention has been developed using the planetary gear train as the core for a speed change mechanism for use especially, but not exclusively, with wheeled or electric vehicles. Whilst the invention has particular application for use with electrically propelled vehicles such as electric bicycles, it should be understood that the invention is not limited to this field but can find applications in other fields where there is a requirement for a speed change mechanism.